Fluid can be obtained from different sources, one of which is a pump that receives a fluid from a fluid supply, displaces the fluid with a mechanical means, and provides the fluid to, for example, a conduit. The mechanical means employed by the pump may be a member with a reciprocal motion, such as pistons, peristaltic rotors, or the like. The reciprocal motion periodically displaces the fluid towards the conduit, thereby causing the fluid to flow. Due to the reciprocal motion, the fluid provided by the pump may have pulses that are carried downstream through the conduit. Accordingly, the pulses are sometimes referred to as fluid born noise (“FBN”). Fluid flow with the FBN is commonly referred to as a pulsating flow.
In addition, flow path geometries formed by conduits with bends, surface irregularities, or the like, can introduce vortices due to vortex shedding. For example, even in laminar fluid flows, vortexes might be generated by the flow path geometry. These vortex trains typically have an interval that is proportional to the flow rate. Accordingly, the faster the fluid flow, the faster the perturbations that are generated by the vortex trains. These vortex trains might travel a distance downstream before the flow re-laminarises. In a simplification for sake of discussion, the portion of the fluid flow with the vortex trains may be viewed as a pulsating flow.
The pumps and the conduits with bends and surface irregularities are typically used in fluid control systems with a valve. The valve may be a proportional valve, although many other valves or flow controllers may be employed with the pumps. For example, a proportional valve downstream from the pump may control a flow rate of the fluid with a flow sensor. More specifically, the flow sensor may measure the flow rate of the fluid flowing through the conduit and provide a flow rate signal to the valve. The flow rate signal can be proportional to the measured flow rate of the fluid. Using the flow rate signal, the proportional valve may control the flow rate of the fluid through the conduit.
However, due to the FBN, perturbations, or other disturbances in the fluid flow, the flow rate signal may also include noise. The noise can cause the proportional valve to be unstable. For example, a position of the proportional valve may not correspond to a flow rate set point and, instead, may continuously oscillate about the set point. Accordingly, it is desirable to attenuate the noise in the flow rate signal.
Passive filters can attenuate the noise component if the noise component has a known or constant frequency. However, the noise component's frequency is correlated with the pump speed. For example, as the pump's rotation speed increases, the noise component's frequency also increases. In addition, the flow rate of the fluid flow may also rapidly change due to various reasons, such as a rapid change in the flow rate set point. Due to the rapid change, the flow rate measurement can have some components with a frequency greater than zero. The passive filters may undesirably attenuate the rapid changes in the flow rate measurements, thereby causing inaccurate flow rate measurements.
These and other issues may be resolved by employing adaptive filters. The adaptive filters may use a noise reference to adaptively filter the noise component in the flow rate signal. For example, if the frequency of the noise changes, the adaptive filter can track the frequency using the noise reference. However, the noise reference may need to be an accurate representation of the noise component in the flow rate signal. If the accurate representation could be obtained from a second filter cascaded with the adaptive filter, then the noise component may be attenuated without undesirably canceling other components in the flow rate signal. Accordingly, there is a need for cascaded adaptive filters for attenuating noise in a feedback path of a flow controller.